1. Field of the Invention
The present invention relates to an industrial machine used in measuring or processing a work piece, and to a method for measuring an amount of expansion/contraction of the industrial machine.
2. Description of Related Art
A coordinate measuring machine (industrial machine) used in measuring a work piece includes guidance corresponding to each of three mutually orthogonal axes; a movable body displacing along the guidance; a scale for measuring an amount of displacement of the movable body; and a probe detecting contact with the work piece (measured object). Such a coordinate measuring machine can obtain three-dimensional coordinate values of the probe from the amount of displacement in each of the axis directions. There is a great deal of variety in structural forms of coordinate measuring machines; however, a bridge displacement-type coordinate measuring machine is the most common.
The bridge displacement-type coordinate measuring machine has a structure in which drive guide mechanisms corresponding to each of the three mutually orthogonal axes are serially stacked. Specifically, the bridge displacement-type coordinate measuring machine includes a base, a Y-carriage, an X-slider, and a Z-ram. A Y-rail is fixated to the base, the Y-rail extending in a Y-axis direction. The Y-carriage is formed to have a bridge structure capable of being guided on the Y-rail and displaced over a top surface of the base. The Y-carriage includes a column, a supporter, and an X-beam supported by the column and supporter and extending in an X-axis direction. The X-slider is capable of being guided and displaced along the X-beam. The Z-ram is capable of being guided on a guide provided to the X-slider and displacing in a Z-axis direction. The Z-ram also holds the probe.
The bridge displacement-type coordinate measuring machine measures a position of the probe in each axis direction using an X-scale, a Y-scale, and a Z-scale. For example, the Z-axis direction position is measured by reading a value of the Z-scale, which is fixated to the Z-ram, with a Z-detection device provided to the X-slider. Moreover, a three-dimensional coordinate system configured by the X-scale, the Y-scale, and the Z-scale and provided to the coordinate measuring machine is referred to as a “machine coordinate system.”
In this regard, in Japanese Patent Laid-open Publication No. 2001-021303, a temperature sensor is mounted to each of an X-scale, a Y-scale, and a Z-scale of a bridge displacement-type coordinate measuring machine. Even in a case where the scales expand and contract according to a thermal expansion coefficient inherent to the scale material due to changes in environmental temperature, an amount of expansion/contraction of the scales is corrected by temperature correction. However, when the column or supporter (structural components other than the scales) expands and contracts in the Z-axis direction due to a change in temperature, the Z-detection device provided to the X-slider is displaced in the Z-axis direction because the X-beam guiding the X-slider is supported by the column and the supporter. As a result, an error in the Z-axis direction may arise in the coordinate values of the probe.
Japanese Patent Laid-open Publication No. 2012-053033 discloses an invention conceived with a focus on the noted concern regarding Japanese Patent Laid-open Publication No. 2001-021303. Specifically, in Japanese Patent Laid-open Publication No. 2012-053033, in order to respond to the Z-axis direction displacement of the Z-detection device caused by the expansion/contraction of the column and the supporter due to a change in temperature, a temperature detection sensor is provided to the column and to the supporter to measure the temperature. Also, an estimated value for the amount of Z-axis direction expansion/contraction of the column and the supporter is calculated based on the temperature and the thermal expansion coefficient of the column and the supporter, and corrections are made based on the estimated value.
However, in carrying out the correction method disclosed in Japanese Patent Laid-open Publication No. 2012-053033, the estimated value for the amount of expansion/contraction calculated using the temperature and the thermal expansion coefficient of the column and the supporter does not necessarily reflect reality, and so the estimated value for the amount of expansion/contraction is unreliable.
In other words, the column and supporter of the coordinate measuring machine are large in size and mass, and so also have a large time constant for changes in temperature. Therefore, due to temperature distribution of the column and the supporter, the estimated value for the amount of expansion/contraction is unreliable. For example, in a case where the temperature detection sensor is positioned at a specific portion on a surface of each of the column and the supporter, there is unreliability due to a difference between a surface temperature and an internal temperature of the column and the supporter. In addition, unreliability of an official thermal expansion coefficient of a material forming the column and the supporter and unreliability of the temperature detection sensor both cause unreliability in the estimated value for the amount of expansion/contraction.